One type of treatment for an oil or gas well is referred to as well fracturing or a well “frac.” Typically an operator connects an adapter to the upper end of a wellhead member such as a tubing head and pumps a liquid at a very high pressure down the well to create fractures in the earth formation. The operator will also then disburse beads or other proppant material in the fracturing fluid to enter the cracks to keep them open after the high pressure is removed. This type of operation is particularly useful for earth formations that have low permeability but adequate porosity and contain hydrocarbons, as the hydrocarbons can flow more easily through the fractures created in the earth formation.
The pressure employed during the frac operation may be many times the natural earth formation pressure that ordinarily would exist. For example, during a frac operation the operator might pump the fluid at a pressure of 8,000 to 9,000 psi, whereas the normal pressure in the wellhead might be only a few hundred to a few thousand psi. Because of this, the body of the wellhead and its associated valves typically may be rated to a pressure that is much lower than what is desired for frac operations. While this is sufficient to contain the normal well formation pressures, it is not enough for the fluid pressure used to fracture the earth formation. Thus, the wellhead and associated valves may be damaged during frac operations.
Moreover, because of the proppant material contained in the frac fluid, the frac fluid can be very abrasive and damaging to parts of the wellhead. To allow the operator to use a pressure greater than the rated capacity of the wellhead seals (including the various valves associated with the wellhead) and to protect against erosion resulting from the frac fluid being pumped at high pressure and volume into the well, the operator may employ an isolation sleeve to isolate these sensitive portions of the wellhead from the frac fluid. An isolation sleeve seals between an adapter above the wellhead and the casing or tubing extending into the well. The sleeve isolates the high pressure, abrasive fracturing fluid from those portions of the wellhead that are most susceptible to damage from the high pressures and abrasive fluids used in well fracturing operations. However, even with the use of an isolation sleeve, unacceptable levels of tensile stress may be induced in the hub section of the wellhead. It is desirable to reduce these tensile stresses in the wellhead.